# How can sound travel as a transverse wave?

We know that sound travels by each previous particle transferring its energy to the next successive particle.

We also know that transverse waves are those waves in which particles move about their mean positions normal to the direction of the propagation of the wave.

Now my question is energy can only be transferred by a particle of a medium to another particle if it collides with it. If the particle moves up and down, how is it going to transfer energy to the particle next to it making it impossible for the sound to propagate.

PLS HELP

• Since sound waves are not transverse what do you really ask? Commented Aug 12 at 13:22
• it's not transverse... what would polarized sound sound like?
– JEB
Commented Aug 12 at 14:03
• Hi Arsh Hussain Naqvi, Welcome to Phys.SE. Are you asking about sound in a gas, fluid or solid? Commented Aug 12 at 14:28
• (i) The simplest example of the propagation of energy orthogonal to the initiating motion is a whip. The handle is moved sideways and the wiggle moves along the whip to the tip. (ii) The individual binary collision in air are not head-on; the particles that meet part in distinct angles defined by the differential cross sections of the molecules potential. It's only statistics of a (Maxwellian) gas that gives the impression of 1-directional motion. Commented Aug 14 at 9:01

Sound in air is not a transverse wave.

Elastic waves in a solid can be transverse or longitudinal, and are sometimes called "sound", but the atoms in a solid are attached to one another by chemical bonds and do not need collisions to transfer energy.

Media with very low shear modulus, like fluids, do not support transverse wave motion. Transverse motion is defined by the condition $$\nabla\cdot\vec{u} = 0$$ (where $$\vec{u}$$ is the displacement). In the equations for elasticity or fluid motion, if you impose this condition, transverse waves require a non-zero shear modulus $$\mu$$. Therefore, for $$\mu = 0$$, there is no transverse motion.

• Actually, fluids with sufficiently high viscosity can also transmit transverse waves. Generally not very well, there is an exponential decay with distance, though there are exceptions - dilatant liquids turn nearly solid in response to strong vibration and can thus transmit high-intensity transverse waves, whereas Bingham plastics (which may or may not be considered liquids) are solid in their rest state and can therefore transmit low-intensity transverse waves. Commented Aug 13 at 11:04
• @leftaroundabout Thank you, I did not know that. Commented Aug 13 at 11:17

If we consider sound in air, the air molecules are already moving in random directions. When a vibrating string (for example) makes a sound its vibration gives the molecules a push which will on average push them towards molecules further away. That will also on average push these molecules in the same direction, and so on. It is an average velocity of the molecules that is important.

In some circumstances you can hear some of the random motion of the air molecules. A sea shell held over an ear is a traditional way to do it.

Sometimes people are confused about what happens when sound is produced by a vibration in a guitar string. The vibration in the string is a transverse wave, and that means it is not actually a sound wave. The sound wave is produced when the string (or the vibrating body of the instrument it is attached to) pushes the air molecules. Likewise the vibration of a loudspeaker diaphragm is not actually a sound wave; the sound wave is made in the air by the diaphragm pushing on it.

• The sound from a guitar primarily does not come from the strings Commented Aug 14 at 12:48
• @Harrychink I said in my answer that the vibrating body of the instrument is involved - for a violin or a guitar that is the primary way the sound wave in the air is generated. Direct string-to-air transmission might be primary for an electric guitar with no electricity. I wrote my answer the way I did to try to simplify things for a beginner, and the question is very much a beginner question. I expected more advanced readers to understand this simplification, as you indeed did. Commented Aug 14 at 13:00

Instead of thinking of loose particles in vacuum, better think of small masses attached to their neighbours by little springs, because your matter is cohesive. This model is a first approximation for the molecules in a solid. (This is called the “harmonic oscillator approximation” of a solid.) The energy is then transported along the medium via the springs.

In fact, you focus too much on kinetic energy alone. A many-particle system with kinetic energy alone would be an ideal gas - and in an ideal gas, sound indeed propagates longitudinally (if at all :-) ). However, coupled matter has additional potential energy (or exchange energy, interaction energy). The transverse propagation of sound is a permament exchange of kinetic and potential energy. If there's no damping, then in order to oscillate, the particles must have speed zero at the turning points, i.e. zero kinetic energy. Due to conservation of energy, all energy of that particle at its turning point is potential energy. This potential energy might be due to an external field, or due to interaction energy with other particles. It is precisely this mechanism which lets energy propagate in sound.

The question you were asking is very good, indeed, because it is important to understand the exchange of kinetic and potential energy in oscillations, and many people don't understand that (even have no clue about that)...